1. Field of the Invention
The present invention relates to a screen-printing method in which a circuit board which has already been screen-printed is screen-printed again so that a new print pattern is formed on a previous pattern and a screen-printing apparatus carrying out the above-described method.
2. Description of the Related Art
Screen-printing apparatuses have generally been provided for screen-printing a solder pattern or the like on a circuit board. For example, JP-A-2000-956 discloses such a screen-printing apparatus in which a circuit board transferred below a screen mask is raised and held in contact with the underside of the screen mask. A squeegee brought into contact with an upper surface of the screen mask is moved so that the circuit board is screen-printed.
Defective printing occurs for some cause even when a normal print pattern is obtained by a single time of screen printing. Upon occurrence of such defective printing, a print pattern sometimes lacks of an amount of printing. As a countermeasure, in an inspection step of inspecting the print pattern of a screen-printed circuit board (hereinafter, “printed board”) being transferred out of the screen-printing apparatus, it is determined by an inspection apparatus or an operator that an amount of printing is insufficient. In this case, the operator sometimes puts the printed board to the screen-operating apparatus again so that “re-printing” is carried out to overlap a new print pattern with the previous pattern.
In the re-printing, however, the solder etc. of the print pattern of the printed board is still wet. Accordingly, when the printed board transferred below a screen mask is raised and brought into contact with the underside of the screen mask, contact shock slightly vibrates or oscillates the screen mask, whereupon the shape of the solder etc. of the print pattern becomes misshapen and blurs. As a result, the printing quality is degraded and defective printing occurs. Furthermore, when solder or the like soils the underside of the screen mask, the underside of the screen mask needs to be cleaned frequently, whereupon the productivity is lowered.